Field
The present invention relates to a power supply system and a voltage control method of a fuel cell.
Related Art
In a power supply system that includes a fuel cell and is configured to cause an electric power to be taken out of the power supply system in response to a required electric power from a load (hereinafter may be referred to as load request), the load request may temporarily have a significant decrease even during operation of the power supply system. The power supply system including the fuel cell is generally characterized by that the energy efficiency of the entire system decreases with a significant decrease in amount of electric power generated by the fuel cell. Accordingly, in response to an extremely low load request for the power supply system, a conventional control procedure stops power generation of the fuel cell. In response to the load request, a secondary battery provided along with the fuel cell in the power supply system is activated to output the required electric power.
When power generation of the fuel cell is stopped in a state that hydrogen remains in an anode-side flow path of the fuel cell and oxygen remains in a cathode-side flow path, the fuel cell shows an extremely high open circuit voltage (OCV). An excessive increase in open circuit voltage of the fuel cell results in excessively increasing the electrode potential at an electrode (cathode) of the fuel cell. This causes elution (degradation) of a catalyst to proceed at the cathode and thereby reduces the power generation performance and the durability of the fuel cell.
After stop of power generation of the fuel cell, hydrogen remaining in the anode-side flow path is transmitted through an electrolyte membrane of the fuel cell to the cathode-side flow path, and an oxidation reaction of the transmitted hydrogen proceeds at the cathode. As a result, after some time since stop of power generation of the fuel cell, the open circuit voltage is deceased cathode potential is decreased), due to consumption of oxygen remaining in the cathode-side flow path. In this case, the cathode catalyst is reduced, so that the cathode catalyst is more likely to be eluted with a subsequent rise of the cathode potential. In the case of a significant decrease of the load request, there is accordingly a need to maintain the voltage of the fuel cell (electrode potential) in an adequate range, in order to suppress deterioration of the catalyst.
A proposed method to maintain the voltage of the fuel cell in an adequate range in response to a significant decrease of the load request continues low power generation in the fuel cell even after the significant decrease of the load request (JP 2013-161571A). For example, a proposed method to continue low power generation stops the supply of oxygen to the fuel cell until the output voltage of the fuel cell decreases to reach a lower limit in a predetermined range, and continues the supply of oxygen to the fuel cell until the output voltage that has been decreased to the lower limit increases to reach an upper limit in the predetermined range.
The control of repeating the supply and the stop of oxygen to the fuel described above, however, causes the output voltage of the fuel cell to be varied between the lower limit and the upper limit in the predetermined range. In the fuel cell, it is expected that the electrode catalyst is more likely to be eluted at the higher electrode potential. In the case where the electrode potential temporarily decreases and subsequently increases, it is expected that the electrode catalyst is more likely to be eluted at the higher degree of decrease of the electrode potential and the higher degree of subsequent increase of the electrode potential. Accordingly there is a need to not only prevent an extremely high electrode potential and an extremely low electrode potential but suppress a variation in voltage of the fuel cell (electrode potential), in order to suppress deterioration of the electrode catalyst and improve the durability of the fuel cell. During continuation of the state having an extremely low load request, the load request may be varied temporarily as mentioned above. It is desirable to suppress a voltage variation even in such a case. Conventionally, control involving a temporary variation in load request has not been sufficiently examined.